Martin has been developing the series with "Kick-Ass" and "Kingsman: The Secret Service" screenwriter Jane Goldman, with the plot taking place thousands of years before "Game of Thrones." Martin confirmed the news in a blog post Tuesday, referring to the new series by the title "The Long Night."

The eighth and final season of the current show is set to air next year.

Network officials at HBO had previously ordered the development of five prequel projects to continue the franchise, with Goldman's story becoming the first to be officially given the green light for a pilot episode in June.

Martin subsequently revealed one of the proposed series had been axed.

"If you have been following along, you know that we started with four (prequels), and eventually went to five," he shared in a blog post over the summer. "One of those has been shelved, I am given to understand... but that does not mean the others are dead."

"Three more Game of Thrones prequels, set in different periods and featuring different characters and storylines, remain in active development," he continued. "Everything I am told indicates that we could film at least one more pilot, and maybe more than one, in the years to come. We do have an entire world and tens of thousands of years of history to play with, after all. But this is television, so nothing is certain."

Watts has yet to comment on the casting news, but it is the latest TV gig she has lined up - last year, the 50-year-old reunited with her "Mulholland Drive" director David Lynch for a role in his revived "Twin Peaks" show, while she is also set to appear as U.S. newswoman Gretchen Carlson in the forthcoming limited series about disgraced TV mogul Roger Ailes.